Electrostatic discharge (ESD) protection is a common and important requirement for many types of electrical equipment. The familiar "electric shock" experienced by a person touching an object, for example, on a day of low humidity, is just the type of electrostatic discharge which can cause malfunction or damage in any apparatus with sensitive electrical circuitry. A typical discharge caused by touching can be in the range of 10,000 volts, and result in current flow of several amperes for about 1 .mu.sec or less. Such a high voltage applied anywhere to an electrical apparatus designed to be sensitive to small voltages is likely to damage, or at least cause a malfunction of, such an apparatus.
One type of electrical apparatus that is sensitive to electrostatic discharges is a thermal ink-jet (TIJ) printhead. Advanced TIJ printers operate by using logic level voltage signals to operate a large number of very small heating elements, which are used to vaporize liquid ink in a corresponding number of small channels or passageways. Thermal ink jet printers are typically designed to have customer-replaceable printheads or print cartridges; ESD protection is necessary to prevent these items from being damaged during handling and machine insertion. In the circuitry for operating an ink-jet printhead, MOS (metal oxide on silicon) technology is often used. Examples of such printhead designs are described in U.S. Pat. No. 5,010,355 to Hawkins et al., U.S. Pat. No. 4,947,192 to Hawkins et al., U.S. Pat. No. 5,075,250 to Hawkins et al., or U.S. Pat. No. 5,063,655 to Lamey et al. A typical resolution of an ink-jet printer is 300 channels or nozzles per inch, each nozzle having a voltage-actuable heating element associated therewith. Experimental printheads with much higher resolution have also been made. It is thus clear that such an apparatus will be sensitive to the relatively large voltages which may be externally created by a typical electrostatic discharge.
It has been found that a common type of failure associated with electrostatic discharges into a thermal ink-jet printhead occurs around the area of a "bonding pad," through which the printhead is connected to an external electronic device. In a printhead using MOS technology, the inputs of the integrated circuit devices are capacitors whose two electrodes are the MOS transistor device channel and the gate electrode material, usually polysilicon. The two capacitor electrodes are separated by silicon dioxide, which is usually grown on the device channel by high temperature oxidation processes. The MOS transistors use a thin silicon dioxide gate insulator to achieve high performance, and this thin oxide is susceptible to catastrophic, irreversible breakdown if the voltage across the dielectric is raised above 20 to 100 volts, depending on device fabrication details.
Numerous conventional systems are known for ESD protection of semiconductor and MOS devices. In general, such circuits use monolithically integrated protective transistors built into the semiconductor circuit to protect the gates of the devices. These protective transistors are arranged to allow a high voltage static discharge transient to pass to ground prior to reaching the protected circuit. For example, U.S. Pat. No. 4,990,984 to Misu discloses a conventional protective transistor ESD protection device for an integrated circuit. Such systems have a disadvantage which prevents them from being applied as protection of thermal ink jet printhead input terminals. The conventional protective transistor for the circuit has only a small current carrying capacity, so that ESD surges which are encountered in the office environment will overwhelm the protective transistor, allowing a part of the ESD surge to reach the protected circuit or causing failure of the ESD protection circuit itself.
When a typical electrostatic discharge protection device is employed around the edges of the bonding pad, which would typically be in the form of a transistor circuit adapted to short out excessive voltages at the pad, the excess voltage tends not to be distributed evenly when it discharges from the pad, but rather tends to concentrate in localized "hot spots" at various points around the edge of the bonding pad, where the high voltages associated with electrostatic discharge pass through one very small area. The hot spot occurs because current conduction increases with temperature so that local fluctuations in device conductivity become greatly amplified. The resulting concentration of current in a small portion of the device causes heating and alloying or "spiking" of the aluminum metallization through the silicon/diffused surface region. There is therefore a need for a device which avoids this specific type of electrostatic discharge failure.